Check if optional method of GRPCCallOptions are implemented

src/objective-c/GRPCClient/GRPCCall.h

@@ -149,7 +149,7 @@ extern id const kGRPCHeadersKey;
 extern id const kGRPCTrailersKey;
 
 /** An object can implement this protocol to receive responses from server from a call. */
-@protocol GRPCResponseHandler
+@protocol GRPCResponseHandler <NSObject>
 
 @optional
 

src/objective-c/GRPCClient/GRPCCall.m

@@ -149,12 +149,16 @@ const char *kCFStreamVarName = "grpc_cfstream";
           }
           if (headers) {
             dispatch_async(handler.dispatchQueue, ^{
+              if ([handler respondsToSelector:@selector(receivedInitialMetadata:)]) {
                 [handler receivedInitialMetadata:headers];
+              }
             });
           }
           if (value) {
             dispatch_async(handler.dispatchQueue, ^{
+              if ([handler respondsToSelector:@selector(receivedMessage:)]) {
                 [handler receivedMessage:value];
+              }
             });
           }
         }
@@ -171,11 +175,15 @@ const char *kCFStreamVarName = "grpc_cfstream";
               }
               if (headers) {
                 dispatch_async(handler.dispatchQueue, ^{
+                  if ([handler respondsToSelector:@selector(receivedInitialMetadata:)]) {
                     [handler receivedInitialMetadata:headers];
+                  }
                 });
               }
               dispatch_async(handler.dispatchQueue, ^{
+                if ([handler respondsToSelector:@selector(closedWithTrailingMetadata:error:)]) {
                   [handler closedWithTrailingMetadata:self->_call.responseTrailers error:errorOrNil];
+                }
               });
             }
           });
@@ -193,6 +201,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
     if (self->_handler) {
       id<GRPCResponseHandler> handler = self->_handler;
       dispatch_async(handler.dispatchQueue, ^{
+        if ([handler respondsToSelector:@selector(closedWithTrailingMetadata:error:)]) {
           [handler closedWithTrailingMetadata:nil
                                         error:[NSError errorWithDomain:kGRPCErrorDomain
                                                                   code:GRPCErrorCodeCancelled
@@ -200,6 +209,7 @@ const char *kCFStreamVarName = "grpc_cfstream";
                                                                          NSLocalizedDescriptionKey :
                                                                            @"Canceled by app"
                                                                          }]];
+        }
       });
       self->_handler = nil;
     }